Differential trans-impedance amplifier receiver using counter-offset circuitry

ABSTRACT

A system for converting an optical signal into an electrical signal includes at least one differential Trans-Impedance Amplifier (TIA). To minimize (preferably eliminate) DC offset issues at the TIA output, an Input Counter-Offset (ICO) circuit is provided to remove the DC component of the initial optical signal from the input to the TIA. To further maximize the removal of DC offset at the TIA output, an Output Counter-Offset circuit is provided to take DC offset from the TIA output for use as a negative feedback directly to the input of the TIA. Modifications of the present invention are also intended for use with two TIA terminations and with a travelling wave photodiode.

This application is a continuation-in-part of application Ser. No.17/031,429, filed Sep. 24, 2020, which is currently pending. Thecontents of application Ser. No. 17/031,429 are incorporated herein byreference.

FIELD OF THE INVENTION

The present invention pertains generally to systems for convertingoptical signals into electrical signals. More particularly, the presentinvention pertains to electro-optical systems that employ input andoutput counter-offset circuits to minimize noise and interference issuesencountered during operations of a differential Trans-ImpedanceAmplifier (TIA). The present invention is particularly, but notexclusively, useful for providing DC offset values from the output of aTIA as feedback for cancelling DC offset issues at the input of the TIA.

BACKGROUND OF THE INVENTION

It is well known that the photocurrent generated by a photodiode inresponse to a modulated optical signal will include both an AC portion,I_(pd)(AC), and a DC portion, I_(pd)(DC). It is also known that the ACportion I_(pd)(AC) of this photocurrent contains all of the informationcontained in the modulated optical signal. The consequence of this isthat only the AC portion I_(pd)(AC) is actually needed for convertingthe optical signal into an electrical signal. It therefore follows that,for the signal processing purpose of converting an optical signal intoan electrical signal, the DC portion I_(pd)(DC) of the optical signal issuperfluous and is sometimes detrimental.

For receiving and amplifying a photocurrent, Trans-Impedance Amplifiers(TIAs) are commonly used in modern optical receiver designs that employa single photodiode. These DC-coupled TIAs, however, typically requirefeedback control from the TIA output to compensate for the DC offsetthat results when an averaged photocurrent is being received at the TIAinput. This DC offset contributes to a condition commonly referred to asoffset issues. Importantly, it is well known that when offset issues arepresent, the TIA's circuit bias conditions will vary with the offsetcurrent, depending on the optical signal strength; therefore, theperformance and the dynamic operational range of the TIA are degraded.

The same considerations noted above are equally applicable, but moreextensive, when differential Trans-Impedance Amplifiers (TIAs) areconsidered. Particularly, because each differential TIA has two inputports and two output ports, the DC offset issues deserve more scrutiny.For instance, considerations of using counter-offset signals ascorrective feedback input for the differential TIA are relevant.

In light of the above, it is an object of the present invention toprovide counter-offset circuits that compensate for the photocurrent DCoffset issues. Yet another object of the present invention is to useinput counter-offset circuits that incorporate single or multiplephotodiodes with single or multiple TIAs in the optical receiver design.Still another object of the present invention is to provide input andoutput counter-offset circuitry that are easy to assemble, simple touse, and comparatively cost effective.

SUMMARY OF THE INVENTION

In overview, for a base configuration of the present invention theprimary purpose is to provide only the AC portion I_(pd)(AC) of aphotocurrent that is generated by a photodiode as the input to a TIA,defined as an Input Counter-Offset (ICO) circuit. Stated differently,the objective is to counter a DC offset that changes the TIA input biaswith respect to the DC portion I_(pd)(DC). Moreover, in accordance withthe present invention, this objective is accomplished using onlycomponents that are included in a circuit element that is interconnectedentirely between the photodiode and the input port of the TIA.

The present invention recognizes that its purpose can be accomplishedusing either a current sensor or a voltage sensor in the circuitelement. Further, the present invention recognizes that the connectionsbetween the photodiode and the TIA, via the circuit element, can beestablished to include either a current sensor or a voltage sensor inthe circuit element. Further, the present invention recognizes that theconnection between the input port of the TIA and the photodiode can beaccomplished with either the cathode or the anode of the photodiode.

Consequently, there are four different embodiments for a baseconfiguration of the present invention that accomplish the same result,i.e., a pure AC portion I_(pd)(AC) input to the TIA. Two of theseembodiments employ a current sensor where either the cathode or theanode of the photodiode is connected to the TIA input port. Theseembodiments employ a current mirror sensor wherein the photocurrent's DCportion I_(pd)(DC) and its image cancel each other at the TIA input. Theother embodiments of the present invention both employ a voltage sensor.For both of these embodiments a voltage deviation, ΔV, at the TIA input,which is due to the DC portion I_(pd)(DC) of the photocurrent, isidentified by a feedback correction processor. The circuit element thenresponds to this ΔV change with a feedback cancellation current whichcan be added or subtracted to the photocurrent. Specifically, thefeedback cancellation current is adjusted until the DC portionI_(pd)(DC) in the photocurrent is suppressed and a pure AC portionI_(pd)(AC) is present at the TIA input.

It is well known that differential Trans-impedance Amplifiers (TIAs)will increase the bandwidth of an opto-electric signal. They do sohowever, at the expense of increased DC offset issues. Consequently,various systems and methods to suppress these issues are disclosed herewhich include system configurations and methods that use at least onedifferential TIA. Also disclosed are system configurations and methodsusing single photodiodes or travelling wave photodiodes with at leasttwo differential TIA terminals.

For systems that employ a differential TIA for converting an opticalsignal into an electrical signal, as disclosed above, a singlephotodiode that is responsive to an optical signal for generating aphotocurrent having an AC component I_(pd)(AC) and a DC componentI_(pd)(DC) can be used. In this case, however, a differentialTrans-Impedance Amplifier (TIA) having a first input port and a secondinput port, as well as first and second DC outputs, is used. As before,a circuit element as disclosed above is used. With a differential TIA,however, the circuit element functions as an Input Counter-Offset (ICO)wherein the ICO output is received as an input to the “first input portof the TIA. Typically, for this embodiment an Output Counter-Offset(OCO) circuit will also be used to help suppress the DC offset.Structurally, the OCO receives a differential DC output from the TIA todetect any DC offset. The DC offset in the TIA output is then employedas a negative feedback to the “second” input port of the TIA to cancelthe output DC offset.

In detail, an OCO will include a pair of low pass filters forrespectively receiving the first and second DC outputs from the TIA. TheOCO will also include an Operational Amplifier (OPA) for receiving theseDC outputs to quantify a voltage difference, ΔV, for the DC offset. Inturn, this DC offset is used as a negative feedback to the “second”input port of the TIA to suppress DC offset at the input to the TIA.

Preferably, the Operational Amplifier (OPA) will have a differentialoutput wherein a first OPA output is used as a negative feedback to the“second” input port of the TIA. In the same manner, the other OPA outputcan then be established as a negative feedback via the ICO to the“first” input port of the TIA. For this embodiment of the presentinvention the differential TIA will also provide a bias voltage for theICO.

For an embodiment of the present invention which uses two differentialTIAs, a photodiode is used that generates a photocurrent which splitsinto a first photocurrent and a second photocurrent. Each photocurrentwill then have an AC component I_(pd)(AC) and a DC component I_(pd)(DC).In combination with the photodiode, a first differential Trans-ImpedanceAmplifier (TIA) receives the first photocurrent from the photodiode, anda second differential Trans-Impedance Amplifier (TIA) receives thesecond photocurrent from the photodiode.

Further, first and second Input Counter-Offset (ICO) circuits are usedcollectively to remove the DC component I_(pd)(DC) from the first andsecond photocurrents to establish inputs for the first and seconddifferential TIAs. Outputs from the first and second differential TIMare then fed to a differential summer to establish the electricalsignal. Additionally, to cancel the respective DC offset issues for thefirst and second differential TIM, DC outputs from the first and seconddifferential TIM are employed as disclosed above.

The present invention also envisions converting a traveling wavephotocurrent into an electrical signal. For this embodiment a travelingwave photodiode which is responsive to an optical signal for generatinga photocurrent splits into a first photocurrent and a secondphotocurrent. A first transmission line transmits the first photocurrentfrom the traveling wave diode to a first differential TIA, and a secondtransmission line transmits the second photocurrent from the travelingwave diode to a second differential TIA. Importantly, the characteristicimpedance of the traveling wave photodiode is the same as those of thetransmission impedance, and the input impedance of the differential TIM.

Like other embodiments of the present invention disclosed above, therespective DC offsets for the first and second TIM that receive signalsfrom the traveling wave diode are suppressed by respective InputCounter-Offset (ICO) circuits.

For all embodiments of the present invention that involve at least oneTIA, the output DC offset of the first differential TIA and the DCoffset of the second differential TIA are caused by circuit unbalancingdue to process-voltage-temperature variations in the TIA.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features of this invention, as well as the invention itself,both as to its structure and its operation, will be best understood fromthe accompanying drawings, taken in conjunction with the accompanyingdescription, in which similar reference characters refer to similarparts, and in which:

FIG. 1 is a schematic diagram of the general circuitry for the presentinvention;

FIG. 2A is a schematic diagram showing components of the circuitryconnected, in combination, with the input port and the diode bias portof the TIA, when the circuitry is configured with the anode of the diodeconnected to the input port of the TIA, and the circuitry includes acurrent sensor;

FIG. 2B is a schematic of a cancellation circuit having a current mirrorsensor for the configuration of the circuitry shown in FIG. 2A;

FIG. 2C is a schematic of a cancellation circuit, having a currentmirror sensor and an auxiliary circuit to enhance the mirroring accuracyand stability, for the configuration of the circuitry shown in FIG. 2A;

FIG. 3A is a schematic diagram showing components of the circuitryconnected, in combination, with the input port and the diode bias portof the TIA, when the circuitry is configured with the cathode of thediode connected to the input port of the TIA, and the circuitry includesa current sensor;

FIG. 3B is a schematic of a cancellation circuit having a current mirrorsensor for the configuration of the circuitry shown in FIG. 3A;

FIG. 3C is a schematic of a cancellation circuit, having a currentmirror sensor and an auxiliary circuit to enhance the mirroring accuracyand stability, for the configuration of the circuitry shown in FIG. 3A;

FIG. 4 is a schematic diagram of a representative feedback function fora current correction controller which is used when a voltage sensor isincluded in the circuit element of the present invention;

FIG. 5 is a schematic diagram of the present invention employing avoltage sensor when the anode of a photodiode is connected to the inputport of a TIA;

FIG. 6 is a schematic diagram of the present invention employing avoltage sensor when the cathode of a photodiode is connected to theinput port of a TIA;

FIG. 7 is a schematic diagram of a differential TIA employing an InputCounter-Offset (ICO) circuit and an Output Counter-Offset (OCO) circuitfor opto-electrical signals;

FIG. 8 is a schematic diagram of a differential TIA showing details ofthe ICO and OCO circuits referred to in FIG. 7;

FIG. 9 is a schematic diagram of a differential TIA employingdifferential feedback elements;

FIG. 10 is a schematic diagram of an optical receiver with two TIAterminations; and

FIG. 11 is a schematic diagram of a travelling wave optical receiverwith two TIA terminations.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring initially to FIG. 1, a circuitry in accordance with thepresent invention is shown and is generally designated 10. As shown, thecircuitry 10 includes a circuit element 12 that is connected between aphotodiode (PD) 14 and a Trans-Impedance Amplifier (TIA) 16. Also, asshown, the photodiode 14 has two nodes, an anode 18 and a cathode 20.Further, the TIA 16 includes an input port 22 and a diode bias port 24.In accordance with the present invention the anode 18 and the cathode 20of the photodiode 14 are connected to the circuit element 12. Also, theinput port 22 and the diode bias port 24 of the TIA 16 are connected tothe circuit element 12.

Still referring to FIG. 1 it will be appreciated that the circuitelement 12 includes a low pass filter (LPF) 26 and a sensor 28 that worktogether to control a current source 30. For purposes of the presentinvention, it is this current source 30 which outputs a cancellationcurrent that cancels the unwanted offset issues that are mentioned abovein the Background of the Invention, which would otherwise arise at theinput port 22 of the TIA 16.

As disclosed in greater detail below, there are several embodiments forthe present invention. Individually, these embodiments differstructurally from each other in two important respects. In one, theorientation of the anode 18 and cathode 20 of the photodiode 14 with thecircuit element 12 can be reversed. In the other, the sensor 28 that isused for the circuit element 12 can be either a current sensor or avoltage sensor. Thus, there are essentially four different embodimentsof the present invention (FIGS. 2A, 3A, 5 and 6, respectively).

FIG. 2A shows an embodiment of the circuit element 12 wherein the anode18 of the photodiode 14 is connected to the input port 22 of the TIA 16and the cathode 20 is connected to its diode bias port 24 through acurrent sensor 32. For the embodiment shown in FIG. 2A, a cancellationcircuit 34 includes a current sensor 32 (the sensor 28 in FIG. 1) thatinteracts with a current source 30. Also, an AC bypass capacitor 36 isconnected between the cathode 20 of the photodiode 14 and the currentsensor 32. With reference to FIG. 2B it will be seen that thecancellation circuit 34 for this embodiment includes a first filteringmirror 38 a that is biased by a voltage V_(b+). In detail, the firstfiltering mirror 38 a, having a current sensor 32 and a low pass filter26 a, is connected to the cathode 20 of the photodiode 14 with the ACbypass capacitor 36 connected therebetween. The second filtering mirror38 b, having a second low pass filter 26 b and a current source 30, isthen connected to the anode 18 of the photodiode 14, with the input port22 of the TIA 16 connected therebetween. The function of AC bypasscapacitor 36 is to direct an AC portion I_(pd)(AC) of the photocurrentfrom cathode 20 to ground, and to direct a DC portion I_(pd)(DC) of thephotocurrent to go through the current sensor 32. Further rejection ofthe AC portion I_(pd)(AC) in the current sensor 32 is accomplished withthe low pass filter 26, e.g., connecting a resistor between the drainand the gate of a metal-oxide-semiconductor current sensor 32 togetherwith a shunt capacitor connecting the sensor's gate to ground. In thisinvention, a low pass filter having a large RC time constant can bemonolithically integrated with an ultra-low leakage metal-oxide gate. Ina cooperation well known in the pertinent art, the current I_(pd)(DC)(referring to FIGS. 2A and 2B) sensed by the first filtering mirror 38 awill be imaged by the second filtering mirror 38 b to thereby create acancellation current which will cancel the DC portion I_(pd)(DC) of thephotocurrent prior to inputting the AC portion I_(pd)(AC) into the TIA.The second low pass filter 26 b in the second filtering mirror is usedto reduce the current noise generated by the current source 30. Analternative cancellation circuit 34 can comprise an auxiliary circuit 48between filtering mirrors 38 a and 38 b, as shown in FIG. 2C. Thisalternative cancellation circuit 34 is provided to improve the currentmirroring accuracy and stability over a wide photocurrent operationalrange.

As a generalized mirror image of the circuit element 12 shown in FIG.2A, FIG. 3A shows an embodiment of the circuit element 12 wherein thecathode 20 of the photodiode 14 is connected to the input port 22 of theTIA 16, and the anode 18 is connected to the diode bias port 24 of theTIA 16. Again, the sensor 28 (in FIG. 1) is a current sensor 32 thatinteracts with a current source 30. In this embodiment, however, the ACbypass capacitor 36 is connected between the anode 18 of the photodiode14 and the current sensor 32. Further, with reference to FIG. 3B it willbe seen that the first filtering mirror 38 a is biased by a voltage Vand is connected to the anode 18 of the photodiode 14, with the ACbypass capacitor 36 connected therebetween. The second filtering mirror38 b is then connected to the cathode 20 of the photodiode 14 and isbiased with a voltage V_(b+), with the input port 22 of the TIA 16connected therebetween. Thus, similar to the embodiment shown in FIG.2A, the current I_(pd)(DC) in the first filtering mirror 38 a will beimaged by the second filtering mirror 38 b to thereby create acancellation current which will cancel the DC portion I_(pd)(DC) in thephotocurrent prior to inputting the AC portion I_(pd)(AC) into the TIA.The current mirroring accuracy and stability, over a wide photocurrentoperational range, can be improved with an alternative cancellationcircuit 34 comprising an auxiliary circuit 48 between filtering mirrors38 a and 38 b shown in FIG. 3C.

FIG. 4 shows a configuration for the present invention wherein voltagemeasurements function in combination with a current source 30. Further,for this configuration using a voltage sensor 40 (see FIG. 5), thecircuit element 12 receives a bias voltage V_(b+) from the diode biasport 24 of the TIA 16 that depends on the orientation to the anode 18and the cathode 20 of the photodiode 14 with the circuit element 12.

As shown in FIG. 5, the anode 18 of the photodiode 14 is connected tothe input port 22 of the TIA 16. On the other hand, the cathode 20 ofthe photodiode 14 is connected to an AC bypass capacitor 36. Further,the circuit element 12 is connected to the diode bias port 24 with abias voltage V_(b+). Within the cancellation circuit 42 of the circuitelement 12, the voltage sensor 40 is connected to a correction processor44. Further, the voltage sensor 40 is connected via a high impedance lowpass filter 26 a to the anode 18 of the photodiode 14 and also to theinput port 22 of the TIA 16.

In combination, the voltage sensed by the voltage sensor 40 from theanode 18 of the photodiode 14 is provided as an output 46 that is sentto the correction processor 44, where a reference voltage, V_(ref), isalso received by the correction processor 44. In the correctionprocessor 44, the difference between the output 46 from the voltagesensor 40 and the reference voltage V_(ref) is identified as adifferential ΔV. This ΔV then generates a correction voltage, connectedthrough a low pass filter 26 b, for adjusting a cancellation currentoutput from the current source 30. As in the other embodiments for thepresent invention, the resultant cancellation current is used forcontrolling any offset issues occurring at the input port 22 of the TIA16.

FIG. 6 shows a comparable configuration for a voltage sensor 40 versionof the circuit element 12. In the configuration shown in FIG. 6,however, the cathode 20 of the photodiode 14 is connected to the inputport 22 of the TIA 16, while the anode 18 of the photodiode 14 isconnected to the AC bypass capacitor 36. Further, the circuit element 12is connected to the diode bias port 24 with a bias voltage V_(b−). Inall other respects the embodiments of the present invention disclosed inFIGS. 5 and 6 function similarly.

In an operation of the present invention, the photodiode 14 generates aphotocurrent in response to an optical signal. As a consequence, thephotocurrent has an AC portion I_(pd)(AC) and a DC portion I_(pd)(DC).As noted above, the purpose of the present invention is to eliminate theDC portion I_(pd)(DC) from the photocurrent as it enters the input port22 of the TIA 16. As also noted above, this can be done in accordancewith the operation of any one of four different configurations for acircuit element 12.

A simplified operation of the embodiments for the circuit element 12shown in FIGS. 2A and 3A which use a current sensor 32, can be explainedby considering the photocurrent I_(pd)(AC)+I_(pd)(DC) that is generatedby the photodiode 14. For these embodiments filtering mirrors 38 a and38 b are used to create an image of the DC portion I_(pd)(DC) of thephotocurrent from one node of the diode 14. This image current is thenfed back into the other node of the diode 14 to cancel (suppress) the DCportion I_(pd)(DC) of the photocurrent prior to its input into the TIA.

Similarly, an operation of embodiments for the circuit element 12 shownin FIGS. 5 and 6 which use a voltage sensor 40, can be explained byagain considering the photocurrent I_(pd)(AC)+I_(pd)(DC) that isgenerated by the photodiode 14. For these embodiments, low pass filters26 a and 26 b are used to isolate the voltage sensor 40 and thecorrection processor 44 from the AC portion I_(pd)(AC) of thephotocurrent. In this isolation, the output 46 of the voltage sensor 40is compared with a reference voltage V_(ref) to identify a differentialvoltage ΔV. This differential voltage ΔV is then used to adjust acancellation current until the DC portion I_(pd)(DC) of the photocurrentis suppressed and only the AC portion I_(pd)(AC) of the photocurrent isprovided for input to the input port 22 of the TIA 16.

Prior disclosure has pertained generally to TIAs having a single inputfor a photocurrent and a single output for an electrical current. Theuse of a differential TIA 50 for a similar purpose is now disclosed.With reference to FIG. 7 it will be seen that a differential TIA has abias port 52, a first input port 54, and a second input port 56. Also,in addition to gain amplifiers, the driver amplifier of differential TIA50 has a first output port 58 and a second output port 60.

FIG. 7 also shows that the bias port 52 and first input port 54 of thedifferential TIA 50 are connected to an Input Counter-Offset (ICO)circuit 62 which, in turn, is connected to the photodiode 14. It is alsoimportant to note that the ICO 62 is essentially the same as the circuitelement 12 disclosed above. FIG. 7 also shows that the output ports 58and 60 of the differential TIA 50 are connected to an OutputCounter-Offset (OCO) circuit 64. Importantly, the output of the OCO 64is connected via a shunt capacitor 66 to the second input port 56 of thedifferential TIA 50.

Referring now to FIG. 8, it is seen that the first output port 58 of thedifferential TIA 50 is connected to a first low pass filter 68 and,likewise, the second output port 60 is connected to a second low passfilter 70. The low pass filters 68 and 70 are then connected with anOperational Amplifier (OPA) 72. Output from the OPA 72 is shownconnected, in sequence, with a third low pass filter 74, a currentsource 76 and a shunt capacitor 78 before connecting with the secondinput port 56 of the differential TIA 50. In combination, the low passfilters 68 and 70, the Operational Amplifier (OPA) 72, the low passfilter 74, current source 76 and shunt capacitor 78 collectivelyconstitute the Output Counter-Offset (OCO) 64 shown in FIG. 7. Thus, theOCO 64 provides a DC current I_(bias)±I_(OCO), to the second input port56 of the differential TIA 50, where I_(bias) provides a TIA input biascurrent and loco is the output DC-offset compensation current.

In contrast with the circuitry shown in FIG. 8 for separate operationsof the ICO 62 and OCO 64, FIG. 9 shows another embodiment of thiscircuitry wherein the ICO 62 and OCO 64 are combined. In detail, forthis embodiment the OPA 72 has an additional differential output 82which is connected via a fourth low pass filter 84 to the current source80. The current source 80 then directs a combined bias I_(bias) and OCOcurrent ∓I_(OCO) to the ICO 62. In the ICO 62, current from the currentsource 80 is combined with the AC component I_(pd)(AC) from thephotodiode 14 for feedback to the first input port 54 of thedifferential TIA 50. Thus, an additional counter-offset is provided forthe first input of the differential TIA 50.

In FIG. 10 an embodiment for an optical receiver is shown and generallydesignated 86 which includes two differential TIAs 50′ and 50″ for thepurpose of providing the optical receiver 86 with a broader bandwidth.Specifically, the photodiode 14 is connected to both an ICO 62′ and anICO 62″. In this combination, the respective input ports 54′ and 54″ ofthe ICO 62′ and the ICO 62″ are each respectively connected to thedifferential TIA 50′ or 50″, to counter DC offset issues. Further, anOCO 64′ interconnects the output ports 58′ and 60′ of the differentialTIA 50′ with its second input port 56′. Also, an OCO 64″ interconnectsthe output ports 58″ and 60″ of the differential TIA 50″ with its secondinput port 56″. Thus, DC offset issues for both differential TIM 50′ and50″ are suppressed while providing a collectively wider photo receiverbandwidth for the differential summer 88.

FIG. 11 shows an optical receiver, generally designated 90, whichaddresses the issues involved when a pair of differential TIM 50′ and50″ are connected with a traveling wave photodiode 92. As shown, atransmission line 94′ interconnects the photodiode 92 with ICO 62′ whileanother transmission line 94″ interconnects the photodiode 92 with ICO62″. In this arrangement, a critical aspect of the optical receiver 90is that there be an impedance match between the differential TIM 50′ and50″. Stated differently when there is an impedance Z₀ for photodiode 92,the impedance of the transmission line 94′ Z_(t) must equal Z₀, and theimpedance of the ICO 62′ Z_(in) must also equal Z₀. Likewise, theimpedance of the transmission line 94′ Z_(t) must equal Z₀, and theimpedance of the ICO 62″ Z_(in), must also equal Z₀. Stated differently,in both directions from the photodiode 92 there must be matchingimpedances: Z₀=Z_(t)i=Z_(in).

While the particular Differential Trans-Impedance Amplifier ReceiverUsing Counter-Offset Circuitry as herein shown and disclosed in detailis fully capable of obtaining the objects and providing the advantagesherein before stated, it is to be understood that it is merelyillustrative of the presently preferred embodiments of the invention andthat no limitations are intended to the details of construction ordesign herein shown other than as described in the appended claims.

What is claimed is:
 1. A system for converting an optical signal into anelectrical signal which comprises: a photodiode responsive to an opticalsignal for generating a photocurrent having an AC component I_(pd)(AC)and a DC component I_(pd)(DC); an Input Counter-Offset (ICO) circuit forremoving the DC component I_(pd)(DC) from the photocurrent to establishan ICO output including the AC component I_(pd)(AC); a differentialTrans-impedance Amplifier (TIA) having a first input port and a secondinput port, wherein the ICO output is received as an input to the firstinput port of the TIA; and an Output Counter-Offset (OCO) circuit forreceiving a differential DC output from the TIA to detect any DC offsettherein, wherein any DC offset in the TIA output is employed as anegative feedback to the second input port of the TIA to cancel theoutput DC offset.
 2. The system of claim 1 wherein the output DC offsetis caused by circuit unbalancing due to process-voltage-temperaturevariations in the TIA.
 3. The system of claim 1 further comprising an ACshunt capacitor located between the OCO and the second input port of theTIA to filter AC noise from the DC output of the TIA.
 4. The system ofclaim 1 wherein the differential TIA has a first DC output and a secondDC output, and wherein the OCO comprises: a first low pass filter forreceiving the first DC output from the TIA; a second low pass filter forreceiving the second DC output from the TIA; and an OperationalAmplifier (OPA) for receiving the first DC output from the first lowpass filter and the second DC output from the second low pass filter toquantify a voltage difference, ΔV, for the DC offset to be used as anegative feedback to the second input port of the TIA to counter theoutput DC offset.
 5. The system of claim 4 wherein the OperationalAmplifier (OPA) has a differential output, and wherein a first OPAoutput is established with a negative feedback to the second input portof the TIA, and a second OPA output is established with a negativefeedback via the ICO to the first input port of the TIA.
 6. The systemof claim 1 wherein the TIA provides a bias voltage for the ICO.
 7. Asystem for converting an optical signal into an electrical signal havingan improved broader bandwidth which comprises: a photodiode responsiveto an optical signal for generating a photocurrent which splits into afirst photocurrent having an AC component I_(pd)(AC) and a DC componentI_(pd)(DC) and a second photocurrent having an AC component I_(pd)(AC)and a DC component I_(pd)(DC); a first differential Trans-impedanceAmplifier (TIA) having a first input port for receiving the firstphotocurrent from the photodiode, and having a second input port, afirst output port and a second output port; a second differentialTrans-Impedance Amplifier (TIA) having a first input port for receivingthe second photocurrent from the photodiode, and having a second inputport, a first output port and a second output port; a first InputCounter-Offset (ICO) circuit for removing the DC component I_(pd)(DC)from the first photocurrent to establish a first ICO output includingits AC component I_(pd)(AC) wherein the first ICO output is received asan input to the first input port of the first differential TIA; a secondInput Counter-Offset (ICO) circuit for removing the DC componentI_(pd)(DC) from the second photocurrent to establish a second ICO outputincluding its AC component I_(pd)(AC) wherein the second ICO output isreceived as an input to the first input port of the second differentialTIA; and a differential summer for receiving the output from the firstDC-coupled output port of the first differential TIA and the output fromthe second DC-coupled output port of the first differential TIA, and forsimultaneously receiving the output from the first DC-coupled outputport of the second differential TIA and the output from the secondDC-coupled output port of the second differential TIA to establish theelectrical signal.
 8. The system of claim 7 further comprising: a firstOutput Counter-Offset (OCO) circuit for receiving the first output andthe second output from the first differential TIA to detect any DCoffset therebetween, wherein any DC offset in the output of the firstdifferential TIA is employed as a negative feedback to the second inputport of the first differential TIA to cancel the output DC offset; and asecond Output Counter-Offset (OCO) circuit for receiving the firstoutput and the second output from the second differential TIA to detectany DC offset therebetween, wherein any DC offset in the output of thesecond differential TIA is employed as a negative feedback to the secondinput port of the second differential TIA to cancel the DC offset. 9.The system of claim 8 wherein the output DC offset of the firstdifferential TIA and the DC offset of the second differential TIA arecaused by circuit unbalancing due to process-voltage-temperaturevariations in the TIA.
 10. The system of claim 8 further comprising: afirst AC shunt capacitor located between the first OCO and the secondinput port of the first TIA to filter AC noise from the DC output of thefirst TIA; and a second AC shunt capacitor located between the secondOCO and the second input port of the second TIA to filter AC noise fromthe DC output of the second TIA.
 11. The system of claim 8 wherein thefirst and second differential TIAs respectively have a first DC outputand a second DC output, and wherein the respective OCOs of the first andsecond differential TIAs each comprises: a first low pass filter forreceiving the first DC output from the TIA; a second low pass filter forreceiving the second DC output from the TIA; and an OperationalAmplifier (OPA) for receiving the first DC output from the first lowpass filter and the second DC output from the second low pass filter toquantify a voltage difference, ΔV, for the DC offsets to be used as anegative feedback to the second input port of the respective TIA. 12.The system of claim 11 wherein the Operational Amplifier (OPA) of eachTIA has a differential output, and wherein a first OPA output isestablished with a negative feedback to the second input port of therespective TIA, and a second OPA output is established with a negativefeedback via the respective ICO to the first input port of therespective TIA.
 13. The system of claim 7 wherein at least onedifferential TIA provides a photodiode bias voltage through the ICO. 14.The system of claim 7 further comprising: a first inductor positionedbetween the photodiode and the first input port of the firstdifferential TIA; and a second inductor positioned between thephotodiode and the first input port of the second differential TIA. 15.A system for converting a traveling wave photocurrent into an electricalsignal which comprises: a traveling wave photodiode responsive to anoptical signal for generating a photocurrent which splits into a firstphotocurrent and a second photocurrent; a first differentialTrans-Impedance Amplifier (TIA) having a first input port for receivingthe first photocurrent from the traveling wave photodiode, and having asecond input port, a first DC output port and a second DC output port; asecond differential Trans-Impedance Amplifier (TIA) having a first inputport for receiving the second photocurrent from the traveling wavephotodiode, and having a second input port, a first DC output port and asecond DC output port; a first transmission line for transmitting thefirst photocurrent from the traveling wave diode to the firstdifferential TIA; and a second transmission line for transmitting thesecond photocurrent from the traveling wave diode to the seconddifferential TIA.
 16. The system of claim 15 wherein a characteristicimpedance of the traveling wave photodiode, an impedance of the firstand second transmission lines, and an impedance of the inputs to thefirst and second differential TIM have the same value.
 17. The system ofclaim 15 further comprising: A first Output Counter-Offset (OCO) circuitfor receiving a differential DC output from the first differential TIAto detect any DC offset therein, wherein any DC offset in the firstdifferential TIA output is employed as a negative feedback to the secondinput port of the first differential TIA to cancel the output DC offset;and a second Output Counter-Offset (OCO) circuit for receiving adifferential DC output from the second differential TIA to detect any DCoffset therein, wherein any DC offset in the second differential TIAoutput is employed as a negative feedback to the second input port ofthe second differential TIA to cancel the output DC offset.
 18. Thesystem of claim 17 wherein the output DC offset of the firstdifferential TIA and the DC offset of the second differential TIA arecaused by circuit unbalancing due to process-voltage-temperaturevariations in the TIA.
 19. The system of claim 18 wherein the first andsecond differential TIM respectively have a first DC output and a secondDC output, and wherein the respective OCOs of the first and seconddifferential TIAs each comprises: a first low pass filter for receivingthe first DC output from the TIA; a second low pass filter for receivingthe second DC output from the TIA; and an Operational Amplifier (OPA)for receiving the first DC output from the first low pass filter and thesecond DC output from the second low pass filter to quantify a voltagedifference, ΔV, for the DC offsets to be used as a negative feedback tothe second input port of the respective TIA.
 20. The system of claim 19wherein the Operational Amplifier (OPA) of each TIA has a differentialoutput, and wherein a first OPA output is established with a negativefeedback to the second input port of the respective TIA, and a secondOPA output is established with a negative feedback via the respectiveICO to the first input port of the respective TIA.